Author Topic: Ignition systems  (Read 6103 times)

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Offline paulcaTopic starter

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Ignition systems
« on: September 30, 2021, 10:24:40 am »
For some reason I was trying to work out how a car ignition work and found myself stumbling.  It doesn't help that automotive people use completely different terms for things, like high tension instead of high voltage, condenser rather than capacitor and coil rather than transformer/inductor?

So here's how far I got... a lot of this is probably wrong and left me with questions.

In an old engine, you would see a coil of wire on the crank or the housing and a magnet on the other.  Each time the magnet passes the coil it creates a pulse of current.  That is then stepped up to high voltage and creates the spark across the plug.  The questions are, how is it stepped up and is this type of ignition void of "points" or does the condensor/capacitor hold the charge from the coil pulse and then release it when the points close? 

In a modern engine with CDI ignition I assume the alternator/battery circuit charges the capacitor constantly and electronics decide when to discharge it through the coil to get a spark.

This "coil" they refer to, I get the feeling it can be multiple things, like the generator coil on the crank or the step up transformer... or an inductor.

Can anyone help clarify for me in more "normal" electronics terms?
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Online jpanhalt

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Re: Ignition systems
« Reply #1 on: September 30, 2021, 11:01:04 am »
The magnet passing a coil is a magneto ignition.  Uncommon in most cars today but still used in piston powered aircraft as it will work without a battery.  The original Ford model T ignition used a "buzz" box.  They are still used today for such things as igniting model pulse jets and giving shocks.  There are lots of variations.  The attached picture is the one I use for my pulse jet.

A brief history:
https://www.modeltcentral.com/Model-T-Ford-Electrical-Specifications.html
https://en.wikipedia.org/wiki/Ignition_system

Ohioans are proud of Charles Kettering whose system was the standard automotive system for decades.  Kettering, Ohio is said to be just outside of Dayton, Ohio (see: Wikipedia).
 

Offline Benta

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Re: Ignition systems
« Reply #2 on: September 30, 2021, 11:01:36 am »
The "old" ignition system you mention is called a "magneto ignition". It has the advantage that no outside excitation is necessary (eg, battery). It's still used in lawn mowers etc.

Since the 1950s it's no longer used in cars. Cars today use ignition coil(s), which is basically a flyback transformer. The primary is energised by the 12 V system, and when ignition is desired, the primary circuit is broken, leading the secondary voltage to peak to around 20 kV.

Earlier, you'd have one or more external ignition coils (large cylindrical thingies), today the coils are much smaller and placed directly on the spark plugs, but the principle is the same.

 

Offline nigelwright7557

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Re: Ignition systems
« Reply #3 on: September 30, 2021, 11:32:57 am »
When the points are closed the coil primary charges up.
When points open there is a large change of voltage on primary which is stepped up in the secondary coil and that gives around 25,000 volts pulse.
 

Offline HighVoltage

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Re: Ignition systems
« Reply #4 on: September 30, 2021, 11:56:40 am »
In layman terms on points ignition:

12V system
3 Ohm primary winding of the coil
3 mH primary inductance
1:100 Transformer ratio

When the points are closed, 12V is applied to the primary winding and the current slowly increases until it reached 4A
12V/3Ohm = 4A

At this time you have stored the maximum energy in the coil
W=1/2*L*i^2
W= 1/2*3*4*4 =24mJ (Energy)

Once the points are opened, the magnetic field collapses and produces a voltage of around 300V peak at the primary windings
This 300V is transformed with the transformer ratio of 1:100
300V * 100 = 30.000 Volts peak secondary high voltage

The reality is a little more complicated but the general picture is correct.
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Offline Circlotron

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Re: Ignition systems
« Reply #5 on: September 30, 2021, 12:40:23 pm »
A magneto ignition has many turns of wire on a laminated iron core. Olde timey versions had 1% of the winding short circuited with a set of contacts ("points"). When a magnet on the flywheel moved into position across the iron core the magnetic flux is sort of prevented from entering the core because of the short circuited winding. When the magnet is in the optimum position the contacts are opened and the magnetic flux now rushes through the core and induces a high voltage into the winding that is then fed to the spark plug. Not perfect, but an accurate enough description for this discussion.

Some of my ignition experiments:









 

Offline Ground_Loop

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Re: Ignition systems
« Reply #6 on: October 01, 2021, 12:28:52 am »
Pulse generator, 30 VDC power supply,  IGBT and ignition coil

https://youtube.com/shorts/LPxDsCX5UfY?feature=share
« Last Edit: October 01, 2021, 12:31:20 am by Ground_Loop »
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Offline David Hess

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Re: Ignition systems
« Reply #7 on: October 01, 2021, 03:42:22 am »
Aren't modern electronic ignitions all capacitive discharge?  That would be where a capacitor discharges into the primary with the secondary stepping the voltage up.  The advantage over Kettering ignition is that with the energy stored in the capacitor instead of the coil is that much higher power can be achieved.

Heathkit used to make a CDI kit which worked on a Kettering system with a switch to change between modes.  It only made a noticeable difference when compression was high.

Common lawn engines use magneto ignition also, and I assume portable generators as well.
 

Online NiHaoMike

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Re: Ignition systems
« Reply #8 on: October 01, 2021, 04:25:26 am »
Hyundai/Kia use a two stage conversion design with the first raising the voltage to 300V or so and the second stage is a forward topology boosting the voltage to the final ignition voltage. Not capacitive discharge as such as the bus voltage doesn't fall that much, but it still has the advantage capacitive discharge offers. They were first to market with a midsize car that gets 35 MPG highway without resorting to hybrid technology.

Aircraft ignition systems are indeed mostly capacitive discharge, except for the few that use high frequency inverters direct to the spark plug.
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Offline james_s

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Re: Ignition systems
« Reply #9 on: October 01, 2021, 04:30:36 am »
I don't know much about the truly modern ones, but most of the 80s-90s solid state ignition systems were not CDI because the CDI ignition was notorious for creating large amounts of RF noise. Instead they worked very much like the old Kettering (points & condenser) system except using a transistor in place of the points and a computer to adjust the timing over a range of operating parameters.

The coil in a conventional ignition system can be thought of as a coupled inductor in a boost converter. The switching device closes, placing 12V across the primary which causes a magnetic flux to build up in the iron core. When the switch opens this field collapses resulting in a high voltage pulse (same principal as a boost converter) which is then stepped up according to the turns ratio between the primary and secondary.

The operation of magneto ignition has already been covered, it works very similarly except voltage is induced in the primary by the moving magnet rather than from the 12V electrical system in a vehicle. The points (or more recently solid state equivalent) still work the same way, opening up to interrupt the circuit causing a rapid collapse of the magnetic flux and a resulting high voltage spike in the primary that is coupled to the secondary.
 

Offline james_s

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Re: Ignition systems
« Reply #10 on: October 01, 2021, 04:33:12 am »
Aircraft ignition systems are indeed mostly capacitive discharge, except for the few that use high frequency inverters direct to the spark plug.

Turbine engine ignitions, including those used in aircraft work like that. They are very similar to the spark igniters used on gas cook stoves. Aircraft piston engines almost exclusively use magneto ignition of the same basic design as used during the dawn of powered flight.
 

Offline bdunham7

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Re: Ignition systems
« Reply #11 on: October 01, 2021, 04:52:57 am »
Aren't modern electronic ignitions all capacitive discharge?  That would be where a capacitor discharges into the primary with the secondary stepping the voltage up.  The advantage over Kettering ignition is that with the energy stored in the capacitor instead of the coil is that much higher power can be achieved.

No, actually, coils and their driving transistors have evolved to pretty high energies and CDI really never was all that widespread among OEMs.  The only OEM application I can remember were old Mercedes Benz V8s from the late 60's and early 70's.  There were probably others, but with low-impedance primary coils and high current drivers, they can make very powerful multi-strike ignition systems using coil-per-plug arrangements.
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Offline David Hess

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Re: Ignition systems
« Reply #12 on: October 01, 2021, 04:58:06 am »
Aircraft piston engines almost exclusively use magneto ignition of the same basic design as used during the dawn of powered flight.

Do all piston aircraft still have dual magnetos with two spark plugs per cylinder for redundancy?

As I recall, the test is to change the magneto selector from both to magneto 1 and then magneto 2 and the engine RPM is only suppose to drop by like 100 RPM or something.  But I remember reading about US aircraft on Pacific island bases during World War 2 where they said maintenance was such a problem that aircraft often would not even run without both magnetos selected.
 

Offline james_s

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Re: Ignition systems
« Reply #13 on: October 01, 2021, 05:01:08 am »
Do all piston aircraft still have dual magnetos with two spark plugs per cylinder for redundancy?

As I recall, the test is to change the magneto selector from both to magneto 1 and then magneto 2 and the engine RPM is only suppose to drop by like 100 RPM or something.  But I remember reading about US aircraft on Pacific island bases during World War 2 where they said maintenance was such a problem that aircraft often would not even run without both magnetos selected.

All certified aircraft engines that I'm familiar with have dual ignition systems with two separate spark plugs per cylinder. I don't think there has been much development there in decades, most of the engines being made are still the same basic designs developed around the 1950s. There are some non-certified automotive engine based aircraft engines that do not, I'm not sure whether these use a magneto or conventional automotive ignition system.
 

Offline bdunham7

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Re: Ignition systems
« Reply #14 on: October 01, 2021, 05:05:18 am »
Do all piston aircraft still have dual magnetos with two spark plugs per cylinder for redundancy?

As I recall, the test is to change the magneto selector from both to magneto 1 and then magneto 2 and the engine RPM is only suppose to drop by like 100 RPM or something.  But I remember reading about US aircraft on Pacific island bases during World War 2 where they said maintenance was such a problem that aircraft often would not even run without both magnetos selected.

All conventional (old style) piston aero engines are like this, yes, but there are examples of newer systems out there flying around.  But they aren't just for redundancy, you lose some power and the engine runs hotter with only one spark plug going.  Some old rotary engines were spark plug fouling monsters, so having dual ignitions helps there too.
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Offline Monkeh

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Re: Ignition systems
« Reply #15 on: October 01, 2021, 05:18:36 am »
Do all piston aircraft still have dual magnetos with two spark plugs per cylinder for redundancy?

As I recall, the test is to change the magneto selector from both to magneto 1 and then magneto 2 and the engine RPM is only suppose to drop by like 100 RPM or something.  But I remember reading about US aircraft on Pacific island bases during World War 2 where they said maintenance was such a problem that aircraft often would not even run without both magnetos selected.

All conventional (old style) piston aero engines are like this, yes, but there are examples of newer systems out there flying around.  But they aren't just for redundancy, you lose some power and the engine runs hotter with only one spark plug going.  Some old rotary engines were spark plug fouling monsters, so having dual ignitions helps there too.

Even relatively recently, some otherwise normal inline car engines have had dual spark for a cleaner burn, and let me tell you, those plugs go on for a while.
 

Offline Circlotron

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Re: Ignition systems
« Reply #16 on: October 01, 2021, 07:44:16 am »
I have made ignition systems for a living for the last 27 years.
One of the most overlooked issues with a capacitor discharge ignition is it's woefully low efficiency, at least when using conventional coils. The short answer is - a 1uF capacitor charged to 450VDC has 100mJ of energy. Discharge it into a coil primary, and once the spark in a running engine has begun it takes about 1500V to maintain the spark. Under these conditions a modern E-core coil will send about 35mJ to the spark gap. An older points type canister coil delivers about 25mJ. if you ignore plug lead resistance the majority of the losses are in the coil secondary. with a 1uF 450V CDI the coil secondary current will be close to 400mA peak. If the coil secondary resistance is say 6000 ohms then there will be 1500V across the spark gap and 2400V lost in the coil secondary resistance. So right way we can see the efficiency would be only 38.5%. One particular inductive ignition I make delivers a measured 150mJ to the spark gap. This is a huge jump compared to a CDI.

I posted a thread about this to a high end car forum about this a while back.

https://www.speed-talk.com/forum/viewtopic.php?t=42274
 
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Offline Circlotron

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Re: Ignition systems
« Reply #17 on: October 01, 2021, 07:50:46 am »
Best thing you can do of course is have a CDI initiate the spark and before it goes out have an inductive ignition jump in and maintain the spark.

https://www.speed-talk.com/forum/viewtopic.php?f=1&t=45340
 

Offline SteveyG

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Re: Ignition systems
« Reply #18 on: October 01, 2021, 08:52:59 am »
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

Coil packs vary in design, but for the most part work the same way, however some will use a wasted spark system whereas others will only fire on the correct cylinder

Ignition coil + points are pulsed by the ignition timing system.
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Offline paulcaTopic starter

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Re: Ignition systems
« Reply #19 on: October 01, 2021, 09:09:53 am »
Interesting, I thought CDI was more common, maybe because I ride motorcycles.  Which use CDI a lot.

Coil packs vary in design, but for the most part work the same way, however some will use a wasted spark system whereas others will only fire on the correct cylinder

Was watching a video about Harley Davidsons.  They really are a pile of junk.  The engineering is many places is, well, really, really poor.  It's not even like they evolved and adapted with the times/tech, they don't/didn't.  And.... are now dieing.

They used a single crank pin for 2 pistons, not because that was a good choice, but because they couldn't be bothered to have two crank pins, balance the engine and have a sane firing interval.  They use a 45* firing interval as a result.  So BANG, BANG................ BANG BANG.  Then because they were cheap skates they decided that per-cylinger ignition was too much effort so they just fire both plugs.  This results in the BANG, BANG, POP.....  The POP is caused by the engine being so poorly made there is enough combustable exhaust gas in the exhaust strike to ignite!

With an unbalanced crank, the bike shakes violently when idleing.  So they mounted the engine on rubber.... with a cheap and nasty steel frame that flexed.  Resulting in many deaths caused by death wobble oscilations.  They added a counter balance (to fix the unbalanced pistons) such that the vibration of the unbalanced mass was forward/back, not up/down, hence the shaking.  The only saving grace is that when moving, some of that insane vibration is muted by the road motion.

I think the video making summed it up by saying, Harley's, you either get them or you don't.  I don't.
« Last Edit: October 01, 2021, 09:13:29 am by paulca »
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Offline HighVoltage

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Re: Ignition systems
« Reply #20 on: October 01, 2021, 09:50:38 am »
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

I am not aware of such system.
Do you have an example of that setup?
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Offline Circlotron

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Re: Ignition systems
« Reply #21 on: October 01, 2021, 10:31:58 am »
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

I am not aware of such system.
Do you have an example of that setup?
I have never heard of such a thing either. I’d like to hear more about this.
 

Offline SteveyG

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Re: Ignition systems
« Reply #22 on: October 01, 2021, 11:25:24 am »
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

I am not aware of such system.
Do you have an example of that setup?

A standard coil on plug is this type, 12V is applied all the time, there is a separate trigger pin. Search coil on plug and you'll see these types.
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Online langwadt

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Re: Ignition systems
« Reply #23 on: October 01, 2021, 11:54:24 am »
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

I am not aware of such system.
Do you have an example of that setup?

A standard coil on plug is this type, 12V is applied all the time, there is a separate trigger pin. Search coil on plug and you'll see these types.

that's a standard ignition coil, the lowside igbt is just moved from the ECU to the coil

 

Offline SteveyG

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Re: Ignition systems
« Reply #24 on: October 01, 2021, 01:13:39 pm »
General definition would be the "standard ignition coil" as the large cylinder type which would feed the distributor, not a COP type.

Looks like there are various COP configurations though. I think only the 3 pin coils are the type with just a transistor inside.
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